This invention relates to circumferential flow type liquid pump, and more particularly to a circumferential flow type liquid pump used as a fuel pump for pumping a liquid-phase fuel such as gasoline from the fuel of a vehicle equipped with an internal combustion engine.
FIGS. 4 and 5 are sectional views showing a pump which is the same in type as a conventional circumferential flow type liquid pump disclosed by Japanese Published Unexamined Patent Application No. 79193/1985. In these figures, reference numeral 1 designates a pump casing assembly which comprises a pump casing body 2 and a cover 3. The pump casing assembly accommodates an impeller 4 with vanes 5 on its periphery. The impeller 4 is mounted on a central shaft 6 so that it is rotated around the central axis with respect to the pump casing assembly 1.
In the pump casing assembly 1, an arcuate elongated pump flow path 7 with a suction inlet 8 and a discharge outlet 9 at both ends is defined in such a manner that it is extended along the outer periphery of the impeller 4 and receives the vanes 5 of the impeller 4.
The upstream end portion of the pump flow path 7 which is on the side of the suction inlet is formed into an enlarged flow path 7a having a predetermined length which is larger in section than the remaining portion, and accordingly lower in internal pressure than the latter, and it has a step 7b at the end where its sectional area is decreased in other words, the remaining portion of the pump flow path 7 between the step 7b and the discharge outlet 9 is smaller in sectional area than the enlarged flow path 7a, and accordingly higher in internal pressure than the latter 7a. A small hole, namely, a gas venting hole 14 is formed in the enlarged flow path near the step 7b so that the pump flow path is communicated with the pump casing assembly 1.
The central shaft 6 of the impeller 4 is the rotary shaft of the rotor 16 of an electric motor 15, and it is rotatably supported by bearings 17 and 18 at both ends.
Further in FIG. 4, reference numeral 19 designates an end cover which has a check valve 22 and a liquid outlet 23, and supports a bracket 24.
The pump casing assembly 1 is coupled to the end cover 19 through the yoke 20 of the motor 15. The yoke 20 accommodates the rotor 16, and forms a liquid chamber 21 between the pump casing assembly 1 and the end cover 19 to store a liquid such as a liquid fuel discharged through the discharge outlet 9. Permanent magnets 25 as a serving as s mounted on the inner wall of the yoke. The liquid chamber 21 is communicated with the liquid outlet 23 with the check valve 22 which is provided in the end cover 19. The bracket 24 supports brushes 27 which are held in sliding contact with the commutator 26 of the rotor 16.
The operation of the circumferential flow type liquid pump thus constructed will be described.
As the impeller 4 is rotated clockwise in FIG. 5 by the electric motor 15, a liquid such as a liquid fuel is sucked into the pump flow path 7 through the suction inlet 8. The liquid thus sucked is increased in pressure by the fluid friction resistance which is provided by high speed rotation of the vanes of the impeller, so that it is caused to flow clockwise in FIG. 5 and then flow through the discharge outlet 9 into the liquid chamber 21. On the other hand, when the vanes of the impeller contact the liquid, the latter is partially vaporized, thus forming bubbles in the liquid. The bubbles thus formed are also allowed to flow into the liquid chamber 21. If the bubbles are supplied through the liquid chamber 21 into the internal combustion engine, a variety of difficulties are caused. In order to eliminate these difficulties, the gas venting hole 14 is formed in the enlarged flow path near the step to discharge the bubbles out of the pump casing assembly 1.
In a circumferential flow type liquid pump used as a fuel pump, when bubbles are formed in the pump flow path by vaporization of the fuel and remain therein, so-called "vapor locking" occurs to obstruct the flow of liquid, thus greatly lowering the pumping capacity. In order to overcome this difficulty, in a conventional circumferential flow type liquid pump, as was described above the gas venting hole is formed in the pump flow path to communicate the latter with the outside of the pump casing assembly, so that bubbles formed in the pump flow path by vaporization of the liquid are discharged through the gas tenting hole into the outside of the pump casing assembly.
However, since the gas venting hole is a small hole formed in the bottom of the enlarged flow path, there are various problems. That is, when the vanes of the impeller contact the liquid such as liquid fuel in the pump flow path, bubbles are formed therein, and the bubbles flow along the inner circular periphery of the pump flow path because of the difference between the bubbles and the liquid both in centrifugal force and in specific gravity. Hence, in order to discharge the bubbles out of the pump casing assembly, it is necessary to discharge a large quantity of substantially bubble-free liquid which is present near the bottom of the pump flow path out of the pump casing assembly. Furthermore, since the gas venting hole is a small hole formed in the enlarged flow path as was described before, great flow resistance is induced when the bubbles together with the liquid flow through the small hole.
Furthermore, since the gas venting hole is vertical with respect to the bottom of the pump flow path, the dynamic pressure of the vortex in the pump flow path cannot be utilized in discharging the bubbles out of the pump casing assembly; that is, the bubbles must be discharged only by the static pressure in the pump flow path. Accordingly, when the fuel is vaporized very much, sometimes the bubbles formed by vaporization of the fuel are not discharged from the pump casing assembly; that is, it is difficult to prevent the occurence of vapor locking.